This invention relates to variable bi-phase modulator circuits for microwave signals, particularly but not exclusively to such circuits suitable for use in, for example, reflected power cancellers for frequency modulated continuous-wave (FMCW) radar, or for use in transmit/receive modules for phased-array radar. The invention also relates to variable resistors, particularly but not exclusively for use in such circuits.
UK Patent Application GB 88.28561.4 published as GB-A-2 226 204 on June 20th 1990 describes, in a reflected power canceller, a bi-phase modulator circuit (FIG. 5) comprising a quadrature power divider (62) which has signal input and output ports (72 and 74) and two control ports (68 and 70), and comprises two variable resistors (64 and 66) each having an input port connected to the respective control port (68 and 70). The whole contents of U.S. Pat. No. 4,970,519, the U.S.A. counterpart of GB-A-2 226 204 are hereby incorporated herein as reference material. It should be noted that (although of earlier priority date than the present application) GB 88.28561.4 is not a prior publication.
In the form disclosed in GB-A-2 226 204, each of the two variable resistors is a PIN diode which is partially forward-biased. These PIN diodes form good variable resistors but such diodes cannot be readily fabricated in presently used microwave monolithic integrated circuit technologies.